Polyacrylonitrile-based oxidation fibers have been produced by subjecting a polyacrylonitrile-based fiber to a heat treatment for oxidation in an oxidizing atmosphere of 200 to 300° C.
The reaction taking place in the heat treatment of polyacrylonitrile-based fiber for oxidation is an exothermic reaction wherein oxidation and cyclization take place simultaneously. A heat treatment at a high temperature results in a high reaction rate and a short treatment time. When the heart treatment for oxidation is conducted rapidly, however, the heat generated in the oxidation reaction is accumulated in the fiber and the fiber-inside temperature increases. As a result, an uncontrollable reaction which is accompanied by yarn breakage and firing, tends to be invited.
Further, the heat treatment for oxidation is ordinarily conducted for strands which are each formed as a bundle of a large number of fibers. When a large number of strands are simultaneously subjected to the heat treatment for oxidation for higher production efficiency, it is impossible to obtain oxidation fiber strands at a high temperature in a short time without efficiently removing the generated reaction heat from the fibers, because heat accumulates easily in the strands.
Since the time required for heat treatment for oxidation is long and the energy required therefor is very large, a further improvement in productivity is needed in the step of heat treatment for oxidation.
FIG. 10 is a schematic drawing showing a conventional heat treatment apparatus for oxidation. (A) is a front section, (B) is a side section, and (C) is a top section.
In FIG. 10(A), 52 is a heat treatment apparatus for oxidation. In a heat treatment chamber 54 thereof run plural steps of paths 57a, 57b, 57c, . . . 57x each formed by a large number of strands 56 arranged horizontally. As shown in FIG. 10(B), the strands 56 are returned by given sets of returning rollers 58 provided outside the heat treatment chamber 54 and are fed into the heat treatment chamber 54 repeatedly.
As shown in FIG. 10(B), the strands 56 forming the plural steps of paths leave and enter the heat treatment chamber 54 through the slits 64a, 66a, 66b and 64b respectively formed in the outer wall 60a, inner wall 62a, inner wall 62b and outer wall 60b of the heat treatment apparatus for oxidation.
As shown in FIG. 10(C), inner side walls 68a and 68b are formed at the both sides of heat treatment chamber 54.
In the left half of the heat treatment chamber 54, an outer side wall 69a is formed outside the inner side wall 68a, and a hot air circulation duct 74a is formed between the inner side wall 68a and the outer side wall 69a. As shown in FIG. 10(A), the hot air circulation duct 74a connects an upper duct 70 and a lower duct 72 both of the heat treatment chamber 54.
A heater 76a provided in the hot air circulation duct 74a generates hot air, and the hot air is sent into the upper duct 70 by a fan 78a and further into the heat treatment chamber 54. Then, the hot air passes between the strands 56 running in a path state and is sent downward. At this time, the strands are heat-treated for oxidation. Incidentally, the hot air heats the strands and also has the role of heat removal.
Then, the hot air passes through the lower duct 72 and is sent into the hot air circulation duct 74a. The hot air is heated therein by the heater 76a. This operation is repeated.
In the left half of the heat treatment chamber 54 shown in FIG. 10(C), an outer side wall 69b is formed outside the inner side wall 68b. Between the inner side wall 68b and the outer side wall 69b is formed a heat-insulating air chamber 80a. 
Meanwhile, the right half of the heat treatment chamber 54 shown in FIG. 10(C) is formed skew-symmetrically to the left half. That is, between the inner side wall 68a and the outer side wall 69a is formed a heat-insulating air chamber 80b. Similarly, between the inner side wall 68b and the outer side wall 69b is formed a hot air circulation duct 74b connecting the upper duct 70 and the lower duct 72 both of the heat treatment chamber 54. 76b is a heater and 78b is a fan.
This heat treatment apparatus is covered, at the circumference, with a heat-insulating material for an enhanced heat efficiency.
Even in such a heat-insulating structure, the temperature, for example, in the vicinity of the inner side walls 68a and 68b of the heat treatment chamber 54 is lower than the average temperature inside the heat treatment chamber 54. As a result, the rate of heat treatment for oxidation, of the strands near the inner walls 68a and 68b is low and the heat treatment of strands for oxidation do not take place uniformly. In order to avoid this problem, strands 56 are ordinarily allowed to run about 200 mm apart from the side walls 68a and 68b in ordinary heat treatment apparatuses for oxidation.
Meanwhile, in the heat treatment chamber 54, a large number of strands 56 forming paths may be allowed to run in one zone wherein the strands 56 are arranged uniformly. However, running of paths in a plurality of zones [two zones 59a and 59b in FIG. 10(A)] in place of one zone, with a given gap X taken between two neighboring zones allows easier handling.
For example, when strands forming paths are allowed to run in one zone and when troubles such as fiber breakage and the like occur, the broken piece of fiber coils around a nearby strand, resulting in worsening of trouble and possible spread of the damage to the whole strands. Further, manual operation may be needed for the troubled strands. For these reasons, it is preferred to divide paths into a plurality of zones with a given gap taken between two neighboring zones.
Therefore, in ordinary heat treatment apparatuses for oxidation, strands 56 forming paths are divided into a plurality of zones, the gap between the inner side wall and paths is kept at about 200 mm, a gap of about 200 mm is taken between two neighboring zones, and a heat treatment of strands for oxidation is conducted.
When, in the above heat treatment apparatus for oxidation, strands running in a state of horizontal plural steps of paths arranged vertically are heat-treated for oxidation in the heat treatment chamber, if the number of strands in the heat treatment chamber is increased for higher productivity, hot air receives an increased resistance and the speed of hot air passing through paths is reduced significantly. Resultantly, the strands undergo insufficient cooling. As a result, heat is generated in the strands and, moreover, breakage of fiber due to generated heat occurs. Further, the broken fiber coils around the fiber of other strand, resulting in worsening of trouble. Incidentally, this problem in heat treatment of polyacrylonitrile-based fiber for oxidation may develop into fire being generated. Because of the occurrence of such a serious problem, significant improvement in productivity of oxidation fiber has heretofore been impossible.